Process for preparing alkylpolysiloxanes



PROCESS FOR PREPAR NG ALKYLPOLY- SILOXANES Charles E. Reed, Waterford, N. Y., and John M. Tome, Pittsfield, Mass., assignors to General Electric Com-v pany, a corporation of New York No Drawing. Application March 17,1955", Serial No. 495,053 r 5 Claims. (Cl. 265-4481) ofat least 175 C. in the presence of a zinc halide as a,

catalyst.

Methylpolysiloranes have been prepared in the past by effecting reaction betyeen alkylhalogenosilanes and water. however, under such circumstances, the hydrogen halide formed as a result of the reaction is corrosive and often exercises an undesirable effect on the alkylpolysiloxanes formed unless provision is made for rapid removal of the hydrogen chloride or reducing the activity of the hydrogen halide by means of high dilution with water or by neutralization in the hydrolysis medium. In addition, when employing such hydrolysis techniques, it is often difiicult to remove the last traces of hydrogen halide from the alkylpolysiloxane so that undesirable effects of the presence of the hydrogen halide are obtained during later processing of the alkylpolysiloxanc. Moreover, the hydrolysis reaction is often violent and diflicul-t to control,

and has required modification, for instance, by carryi out the hydrolysis in two or more stages, or else employing expensive and toxic solvents.

U. S. Patent.2,556,8-9.7, issued June 12, 1951, attempts to correct some of these difficulties by eflecting reaction between an alkylhalogenosilane and a lower alkanol, such as methanol, whereby part of the product formed is an alkyl halide yielding the desired alkylpolysiloxane, for instance, the methylpolysiloxane. However, in this patent, it is stated that the temperature of the reaction should be below 60 C. in order to avoid undesirable liberated amounts of hydrogen halide. However, the conditions, under which this reaction is carried out are not conducive to useful processing for several reasons. In the first place, the material balance, i. e., the desirable alkylpolysiloxanes, such as the methylpolysiloxanes, formed, are not produced in good enough yield and the yield of alkyl halide which is expected as a result of the reaction is undesirably poor. Moreover, the time of reaction isunduly long, as evidenced by the fact that the single example cited in this patent states that, after 5 hours, only half the theoretical quantity of alkyl halide had been liberated. Furthermore, contrary to the statement in the patent, it has been found that large amounts of hydrogen halide are still present in the reaction mixture, which requires careful removal or neutralization and also this reaction gives poorer yields ofthe desired product than is indicated in the patent.

We have now'discovered that we can obtain improved yields of alkylpolysiloxane's- [e. g.; cyclic derivatives of the formula (R2SlO)n where R is a lower alkyl radical, for

United States Patent Q instance, methyl, ethyl, propyl, etc., and n is an integer equal to from 3 to 8 or more] over those described in the aforementioned U. S. Patent 2,556,897, that such yields can be obtained in relatively short periods of ti me,that the conversion of the reaction products to the useful alkyl halide which can be re-used, for instance, in making fresh alkylhalogenosilanes can be materially increased, and the amount of hydrogen halide present in the reaction zone. or in the reaction mixture greatly reduced and, in some re,- spects, substantially eliminated. All these desirable results are accomplished by efiecting reaction between the dialkyldihalogenosilane and the alkanol in the vapor phase at a temperature above 175 C. in the presence of a zinc halide, for instance, zinc chloride (ZnCla) as the catalyst. Amongthe dialkyldihalogenosilanes that may be employed in the practice of the present invention are, for instance, dimethyldichlorosilane, diethyldichlorosilane, dipropyldichlorosilane, dimethyldibromosilane, etc. Preferably, the alkyl group of the dialkyldihalogend, silane contains from 1 to 2 carbon atoms and the halogen is chlorine.

The alkanol used for reaction with the dialkyldihalo genosilaue is selected from the lower alkyl monohydric alcohols, as, for instance, methanol, ethanol, propanol. Preferably, the alcohol is methyl alcohol when used with dimethyldichlorosilane, and ethyl alcohol when used with diethyldichlorosilane, because of the fact that the, formed alkyl halide, for instance, methyl chloride when using methanol and ethyl chloride when usingethanol, can be employed forpassage over silicon in the, presence of: copper to form new amounts of the corresponding dialkyl dichlorosilane in accordance with the process described in Rochow patent 2,380,995 issued August 7, 1945 and assigned to the same assignee as the present invention.

The proportion of alkanol to dialkyldihalogenosilane is critical if one is to obtain optimum yields of the alkylpolysiloxane and the alkyl halide. We have found that the concurrent passage of the alkanol and the dialkyldihalogenosilane through the heated zone in the presence of the zinc halide catalyst is preferably carried out under conditions wherein there are present from about 2 to. 3.5 mols methanol per mol of the dialkyldihalogenosilane, Preferably, at reaction temperatures of from about 250 to 375 C., ratios from 2.5 to 3 mols of methanol per mol of dimethyldichlorosilane are advantageously employed.

The temperature at which the reaction is carried outis advantageously maintained at a temperature of from about 175 to 400 C. If temperatures below 175 C. are, used, it will be found that although the yield of desirable alkylpolysiloxane products may be the same, the percentage conversion to the alkyl halide, for instance, methyl chloride, will be lower, while large amounts of hydrogen halide will be found in the reaction product. Optimum temperature ranges are from about 250 to. 375 C. The

upper temperature limit should be below that at which undesirable decomposition of either the reaction products or the reaction ingredients occurs.

The preferred catalyst is zinc chloride, although any zinc halide may be employed. However, it is desirable to employ a zinc halide containing the same halogen as is in the dialkyldihalogenosilane. The carrier for the zinc halide is preferably a silica gel, for example, a silica gel identified as activated silica gel sold by Davison Chemical Corporation, Baltimore, Maryland. This silica gel is an amorphous, extremely porous form of silica having the.

appearance of clear, crushed quartz. It has a high adsorptive capacity for gases and liquids. It'may have a surface area range of from to 850 square meters/L gram, a pore diameter range of from 1 0 to 100 Angstrom units, and a particle density of L20. It can be prepared by adding H2804 to an aqueous solution of sodium silicate,-

to give a gel which is preferably washed to remove sodium sulfate, and then dried to give the silica gel.

The amount of zinc halide in combination with this silica gel may range widely and is preferably within the temperature as a vaporizer preheat unit, and a 40-inch section of catalyst bed maintained again at the requisite temperature for reaction as the reaction site. The exit reaction products were cooled with a water condenser packed with mm. glass beads maintained at the stipulated range from about 2 to 50 per cent, by weight, based on the 5 and those condensed were retained by the receiver while total weight of the silica gel and zinc halide. Amounts those still volatile at room temperature were collected in of zinc chloride in excess of 30 per cent, by weight, of the a Dry Ice-isopropanol trap system. Liquid methanol and total weight of the zinc halide and the silica gel do not dimethyldichlorosilane were fed with rates controlled by show any significant improvement over cases where the Sigmamotor pumps, E and M Enterprises, Inc., Middlezino halide is below 30 per cent, by weight. Use of the port, N. Y., into the heated portion of the glass tube conzinc halide in the lower ranges, although giving accepttaining the glass beads. able alkylpolysiloxane formation, nevertheless may give In these examples, the water-condensable products conlower alkyl halide conversion than is obtainable with the sisted of a layer of methylpolysiloxane and a layer conhigher amounts of the zinc halide. sisting of water, excess methanol and some dissolved hy- Although the zinc halide in moist form may be mixed 5 drogen chloride. The methylpolysiloxane layer was sepatogether with the silica gel, and this dried and employed rated and washed with Water and the yield of methylas a bed over which the vapors of the alkanol and the polysiloxane determined. dialkyldihalogenosilane pass, a more advantageous method comprises dissolving the zinc halide in water and mixing EXAMPLE 1 V the zinc halide solution with silica gel to intimately dis- In this p mixtures 05 VaPOIiZCd dimethyldichkiperse in a homogeneous manner th Zinc chlorid ver rosilane and methanol were passed in various proportions the silica gel, and thereafter drying the mixture to remove Over silk?! 2 impregnated with Varying amounts of Zinc substantially all traces of moisture so that, under reaction chloride in the manner described above. As controls, conditions, substantially anhydrous conditions are emsimilar passages of methanol and dimethyldichlorosilane ployed. were made over the silica gel which did not contain any In carrying out the reaction, the zinc halide and silica Zinc chloride and over silica gel containing another magel are packed into a reaction tube which may or may terial which might be considered a catalyst, such as calnot be preceded by an inactive bed of finely divided acium oxide. Several of the runs carried out were mainterials (which acts as a volatilizing zone), for instance, tained at difierent temperatures to determine the effect of finely divided glass beads, etc. Thereafter, the alkanol t mper re g Th6 following Table 1 Shows the and the dialkyldihalogenosilane vapors are passed through ingredients used, the proportion of such ingredients, the the catalyst bed of the zinc halide-silica gel mixture, maintemperature conditions, and the amounts of product obtaining at all times the desired temperature range in the tained. The vapors of the methanol and dimethyldichloreaction zone. Thereafter, the reaction products as well rosilane were passed through the respective beds within a as unreacted materials may be led into suitable condensspace of about 100 to 130 minutes.

Table 1 Run Number Temperature 300 0 300 0. 300 (3. 225 0. 300 0. 375 0. 300 0. 300 0. Catalyst Silica gel Silica gel Silica gel Silica gel Silica gel Silica gel Silica gel Silica gel alone 10% (33.0 10% Z1101; 10% ZnCh 20% ZnClz 20% 21101: 20% 211012 22% ZnCla Reactants:

Moles Used (omnsioh. 2.18 1. 9a 1. as 1.83 2. 04 1. 0a 2. as 2.22 Moles CH3OH 7. 33 5. 92 5. 31 5. 09 5. 02 4. 94 5. 1s 5. 2s Moles ratio a. 35 1 2. 1 2. 82/1 2. 78/1 3. 24/1 2. 04/1 2. 08/1 2. 82/1 rrm'iiuigiiseot run (minutes) 13 100 100 13 120 120 Methylpolysiloxane (grams) s5. 5 s7. 9 88. 9 11s. a 131. a 07. 7 208. 0 160. 1

Percent recovery of silicon as methylpol siloxane .53. 7 4s. 4 63. 8 s7. 5 s7. 3 s1 94. 77 97. a Moles unreactecl Sl-CL 1. 01 0. 51 0.01 0.05 0. 00s 0. 00a 0. 24 0 Moles ofitcl formed..- 0. 0s 1. 01 2.03 1. 47 1. as 1. 70 2. 27 3. 48 Percent 011301 (of theoretical) 22 25. s 54. 2 40. 2 46.1 5.4 38.1 78. 4

ing traps maintained at various temperatures designed to It will be apparent from the above-mentioned described effect either solidification or liquefaction of the reaction table that increasing the amount of zinc chloride as products or reaction ingredients. catalyst increases the yield of methyl chloride, indicat- In order that those skilled in the art may better under- 60 ing that greater theoretical conversion of the dimethyldi stand how the present invention may be practiced, the chlorosilane to the methylpolysiloxane was being effected. following examples are given by way of illustration and Along these lines, it was found that when the catalyst not by way. of limitation. All parts are by weight. concentration was increased to 30 per cent, or in another In the following examples, the dialkyldihalogenosilane instance to 50 per cent, the results were essentially the employed was dimethyldichlorosilane, and the alkanol same as those obtained by using 28 per cent zinc chloride. was methanol. The silica gel-zinc halide catalyst bed 69 Further observation is the fact that decreasing the temwas prepared by dissolving zinc chloride in water in such perature from 30Q C. to 225 C. had little eli'ect on the amounts that when the zinc chloride-water solution was methylpolysiloxane recovery but decreased the methyl mixed with activated silica gel and the mixture of incltloride formation. Increasing the temperature from gredients dried to remove substantially all thewater, there m 300 C. to 375 C. had the reverse effect, that is, alwas deposited on the activated silica gel (Davison silica gel though it lowered somewhat the yield of methylpoly of about 8 to 14 mesh) in a homogeneous fashion the siloxanes, nevertheless the methyl chloride production recited amount of zinc chloride. was increased; however, this also resulted in increased The reaction zone comprised a one-inch diameter Pyrex formation of dimethyl ether and increased amounts of glass tube containing consecutively a 12-inch section 75 pyrolysis productsof the methylpolysiloxane due to the elevated temperatures.

It is desired to point out that a significant amount (approximately to 30 per cent) of the chlorine from the reaction appears as hydrogen chloride in a methanolwater phase as a condensation product of the exit reaction gases. It was found that by recycling this liquid phase over an alumina (A1203) catalyst reactor, excellent recoveries of methyl chloride were obtained as long as there was no methylpolysiloxane present. Recycling this hydrogen chloride-methanol-water over silica gel containing per cent zinc chloride at 300 C. gave a chlorine conversion of hydrogen chloride to methyl chioride of about 70 per cent. This methyl chloride can then be reacted anew with silicon in the presence of copper to form additional amounts of dimethyldichlorosilane employed in the preparation of the alkylpolysiloxane in accordance with the process described in the present application.

The methylpolysiloxanes obtained in accordance with the directions in the above-identified example comprise about 50 per cent, by weight of cyclic polydimethylsiloxanes, for instance, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, etc. The remainder of the methylpolysiloxane product constitutes higher molecular weight materials than the aforesaid cyclic derivatives and may contain small amounts of intercondensed monomethylsiloxane units (CHsSiO).

The results described in the foregoing example should be contrasted with those obtained by heating a mixture composed of 2 mols of methyl alcohol and 1 mol of dimethyldichlorosilane at the reflux temperature of the mass. Contrary to what might be expected, large amounts of hydrogen chloride were removed but no evidence of methyl chloride formation was noticed.

It will, of course, be apparent to those skilled in the art that other dialkyldihalogenosilanes, many examples of which were mentioned above, as well as other alkanols, may be used in place of those employed in the foregoing example without departing from the scope of the invention. In addition, the catalyst concentration may be varied within wide limits and the temperature of reaction, as well as the molar concentration of the reactants, may be varied within the scope recited previously. The type of activated silica gel may also obviously be varied; and activated silica gels such as those used for gas adsorption or drying agents, as refrigeration dryers, those used in protective packaging, etc. may be employed for the purpose.

The alkylpolysiloxanes, particularly the cyclic polydialkylsiloxanes, obtained in accordance with the practice of the present invention may be employed in various applications, for instance, as lubricants, or they can be used as intermediates in the preparation of various silicone oils, resins and rubbers. Thus, the cyclic polydimethylsiloxane, particularly the octamethylcyclotetrasiloxane, can be condensed with small amounts of the order of about 0.001 to 0.01 per cent, by weight thereof, of an alkali-metal hydroxide, such as potassium hydroxide at temperatures of from about to C. for times ranging from about 1 to 4 hours, to give a highly viscous methylpolysiloxane product which can be compounded with various fillers, for instance, silica aerogel, gamma alumina, titanium dioxide, etc., in amounts ranging from about 50 per cent or more of the weight of the methylpolysiloxane, and vulcanized, employing benzoyl peroxide as a vulcanizing agent to give elastomeric methylpolysiloxane products useful in applications requiring resistance to elevated temperatures for long periods of time and capable of maintaining flexibility at temperatures below 25 to 50 C. The methylpolysiloxane rubbers can be employed as gaskets. They also can be used to make silicone rubber nipples, etc.

Alternatively, the methylpolysiloxanes obtained in accordance with the practice of the present invention can be interacted with, for instance, hexamethyldisiloxane in the presence of small amounts of an alkali metal hydroxide to give organopolysiloxane fluids of a linear nature useful in hydraulic apparatus or as lubricants. The uses of the alkylpolysiloxanes prepared in accordance with the process of the present invention are well known and are readily ascertained in the periodicals and patent literature available in the art.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. The process for making alkylpolysiloxanes which comprises simultaneously passing a dialkyldihalogenosilane and an alkanol, both in the vapor phase, over a catalyst bed composed of a zinc halide and silica gel While maintaining the temperature of the reaction above 175 C. and below the decomposition point of the reactants and the reaction products.

2. The process for making methylpolysiloxanes which comprises simultaneously passing dimethyldichlorosilane and methanol, both in the vapor phase, over a catalyst bed composed of zinc halide and silica gel while maintaining the temperature of the reaction between 175 C. to 400 C.

3. The process as in claim 2 in which zinc halide is zinc chloride.

4. The process for making ethylpolysiloxanes which comprises simultaneously passing diethyldichlorosilane and ethanol, both in the vapor phase, over a catalyst bed composed of zinc halide and silica gel while maintaining the temperature of the reaction between 175 and 400 C.

5. The process as in claim 4 in which the zinc halide is zinc chloride.

References Cited in the file of this patent UNITED STATES PATENTS 2,485,928 Servais Oct. 25, 1949 2,553,845 Clark May 22, 1951 2,556,897 Bidaud June 12, 1951 2,695,307 Guillisen et a1 NOV. 23, 1954 

1. THE PROCESS FOR MAKING ALKYLPOLYSILOXANES WHICH COMPRISES SIMULTANEOUSLY PASSING A DIALKYLDIHALOGENOSILANE AND AN ALKANOL, BOTH IN THE VAPOR PHASE, OVER A CASTALYST BED COMPOSED OF A ZINC HALIDE AND SILICA GEL WHILE MAINTAINING THE TEMPERATURE OF THE REACTION ABOVE 175* AND BELOW THE DECOMPOSITION POINT OF THE REACTANTS AND THE REACTION PRODUCTS. 